This invention relates to brazing alloys and more particularly to an improved alloy paste for brazing together parts such as tungsten carbide tool bits and a supporting steel body or shank.
The brazing together of two dissimilar metals such as tungsten carbide and steel raises the problem of stress being developed in the brazed joint on cooling because of the different coefficients of expansion of the brazed parts. This stress can result in cracks in the joint leading to its ultimate failure. One technique which has been used to alleviate such stress is a three layer solid brazing alloy having a ductile center layer sandwiched between two brazing layers, the center layer having a melting temperature higher than that of the outer layers. On cooling, the ductile center layer absorbs the stress developed by the different contraction rates of the joined members. A multi-layer brazing alloy of this type is described in U.S. Pat. No. 4,340,650.
One of the drawbacks of solid multi-layer preforms of this type is that they must be stamped, cut, blanked or otherwise formed to the many different shapes and sizes of the members being joined, involving many production steps and resulting in a large inventory of preforms. In addition, preforms generally are preplaced between the members manually or by related techniques which are not well suited for automated high-volume production methods. On the other hand, brazing alloy paste is simple to prepare, eliminates inventory problems, and is ideally suited for placement on joints to be brazed through use of automated paste metering equipment. It is highly desirable, therefore, that these advantages of brazing alloy paste be utilized in joining dissimilar metals such as mine tool bits to steel shanks. However, such pastes of the prior art do not have any provision for relieving stress between the dissimilar members upon cooling and therefore are not a satisfactory substitute for the solid multi-layer prefroms for such applications.
Another technique that has been proposed is described in Pat. No. 2,606,362 to Martin, et al. This patent teaches brazing joint thickness control by the addition of 30-35% by volume of inert (tungsten) particles to the joint during the brazing operation and the use of brazing alloy preforms in making the joint. The disadvantages of this technique are the substantial reduction in the quantity of filler alloy available for the joint, the necessity of maintaining an inventory of preforms of different sizes, and the difficulty of handling and placement of the preforms during the brazing operation.
U.S. Pat. No. 2,451,099 of La Motte describes a method of bonding ferrous metal propeller blade parts with foil or powdered filler metal together with steel or nickel spacing balls. The disadvantages of this method are the necessity of maintaining separate inventories of preforms or powder and balls and the difficulty of placement of these parts during brazing of complex shapes.
This invention is directed to a brazing alloy paste which overcomes these problems.
A general object of this invention is the provision of a brazing alloy paste for joining members having different thermal coefficients of expansion.
A further object is the provision of a brazing alloy paste capable of maintaining a predetermined gap thickness between parts being joined.
A more specific object is the provision of a brazing alloy paste useful for high-volume brazing of joints between similar and dissimilar metals.
These and other objects of the invention are achieved with a brazing alloy paste containing a ductile spacer metal comprising spherically shaped particles of predetermined diameter and having a melting temperature greater than that of the brazing alloy. The metal spheres are mixed in the paste at a relatively low concentration to avoid stacking and thereby control the thickness of the joint to approximately the diameter of the spheres while providing a stress-relieving interface, if desired, between the parts being joined.
In preparing a paste in accordance with the principles of this invention, the spacer metal is atomized to produce spherically shaped powder and has a melting temperature sufficiently greater than that of the brazing alloy powder with shich it is mixed that the former does not melt when the alloy melts and flows. In addition, the metals or alloys comprising the brazing alloy and the spacer metal are selected to minimize the alloying of these two components while providing that the spherical particles are wetted by the alloy. Examples of components which meet these conditions are a brazing alloy consisting of Nicuman 37 manufactured by GTE Products Corporation, WESGO Division (nominal composition by weight 9.5% Ni-52.5% Cu-38.0% Mn) or Nicuman 23 (nominal composition by weight 9.0% Ni-67.5% Cu-23.5% Mn) and a spacer metal useful with these alloys selected from a group consisting of copper, and either iron or 1010 steel. Other compositions are a brazing alloy of copper or silver-copper eutectic and a spacer metal of nickel, iron or 1010 steel.
The spherically shaped spacer metal powder is obtained by atomizing. This powder is then passed through the fine screen with a sieve grading selected to pass particles having a predetermined diameter for example 10 mils. The screened powder is then mixed with the brazing alloy powder and a water soluble resin or a hydroxypropyl cellulose in sufficient proportions to provide a paste of desired consistency.
As noted above the ratio of the spherically shaped spacer metal powder to the brazing alloy powder in the mixture is limited in order to insure that the spheres will not stack on top of each other in the braze joint. For this reason the spherical powder content is maintained in the range of 1-8% by weight of the metal components of the paste, a preferred composition being about 5% by weight.